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Posted: Jan 19, 2006

New theory explains electronic and thermal behavior of nanotubes

(Nanowerk News) Researchers at the University of Illinois at Urbana-Champaign
have made an important theoretical breakthrough in the understanding
of energy dissipation and thermal breakdown in metallic carbon nanotubes.
Their discovery will help move nanotube wires from laboratory to marketplace.

The remarkable electrical and mechanical properties of metallic carbon
nanotubes make them promising candidates for interconnects in future
nanoscale electronic devices. But, like tiny metal wires, nanotubes
grow hotter as electrical current is increased. At some point, a nanotube
will burn apart like an element in a blown fuse.

“Heat dissipation is a fundamental problem of electronic transport
at the nanoscale,” said Jean-Pierre Leburton, the Gregory Stillman
Professor of Electrical and Computer
Engineering at Illinois and co-author of a paper published in the
Dec. 21, 2005 issue of the journal Physical Review Letters. “To fully
utilize nanotubes as interconnects, we must characterize them and understand
their behavior and operating limits.”

For example, in both theory and experiment, the shorter the nanotube,
the larger the current that can be carried before thermal breakdown
occurs. Also, the longer the nanotube, the faster the rise in temperature
as the threshold current for thermal heating is reduced.

In nanotubes, heat generated by electrical resistance creates atomic
vibrations in the nanostructure, which causes more collisions with the
charge carriers. The additional collisions generate more heat and more
vibrations, followed by even more collisions in a vicious cycle that
ends when the nanotube burns apart, breaking the circuit.

“Short nanotubes can carry more current before burning apart because
they dissipate heat better than longer nanotubes,” Leburton said.
“Although the entire nanotube experiences resistance heating,
the electrical contacts at each end act as heat sinks, which in short
nanotubes are relatively close to one another, leading to efficient
heat removal.”

This phenomenon also explains why the highest temperature always occurs
in the middle of the nanostructure, Leburton said, “which is the
furthest point away from the two ends, and where burning occurs in longer
nanotubes under electrical stress.”

In another important finding, Leburton and his colleagues have revised
the common belief that charge carriers go ballistic in short metallic
nanotubes having high currents. Researchers had previously thought that
charge carriers traveled from one terminal to the other like a rocket;
that is, without experiencing collisions.

“We have shown that the high current level in short metallic nanotubes
is not due to ballistic transport, but to reduced heating effects,”
Leburton said. “Owing to their large concentration, the charge
carriers collide efficiently among themselves, which prevent them from
going ballistic. Even in short nanostructures, the current level is
determined by a balance between the attractive force of the external
electric field and the frictional force caused by the nanotube thermal
vibrations. The collisions among charge carriers help the energy transfer
to the nanotubes which results in heat dissipation.”